The present invention relates to a thermal transfer image-receiving sheet which is receptive to a dye transferred from a thermal transfer sheet by heating, which thermal transfer image-receiving sheet can be widely utilized in the field of various color printers including video printers.
In recent years, a system where video images, TV images and still images, such as computer graphics, are directly printed as a full color image has advanced, which has led to a rapid expansion of the market thereof.
Among others, a system which has attracted attention is such that a sublimable dye as a recording material is put on an image-receiving sheet and heated by means of a thermal head in response to recording signals to transfer the dye onto the image-receiving sheet, thereby forming a recorded image.
In this recording system, since a dye is used as the colorant, the sharpness is very high and, at the same time, the transparency is excellent, so that it is possible to provide an image having excellent reproduction and gradation of intermediate colors equivalent to those of an image formed by the conventional full color offset printing and gravure printing. In this case, the formed image has a high quality comparable to photographic images.
Printers in current use in the above thermal transfer system are mainly of such a type that a thermal transfer image-receiving sheet is automatically carried to a thermal transfer section within a printer and, after printing, automatically delivered from the printer. Further, in order to carry out overlap printing of three colors or four colors, it is a common practice to provide a detection mark on the thermal transfer image-receiving sheet in its image-unreceptive surface, that is, the back surface, located opposite to the image-receiving surface for the purpose of preventing the occurrence of a shear in the printing position of each color.
Not only the construction of the thermal transfer sheet but also the construction of the image-receiving sheet on which an image is to be formed is important to the practice of the above thermal transfer method with a high efficiency. In particular, the properties of the image-unreceptive surface (back surface) located opposite to the image-receptive surface of the thermal transfer image-receiving sheet are important for smoothly carrying out automatic feed and delivery of the thermal transfer image-receiving sheet.
For example, when the image-receiving sheets with an image being formed thereon are put on top of another for storage, the dye on the print surface migrates to the back surface of another thermal transfer image-receiving sheet in contact with the print surface to remarkably stain the back surface, which deteriorates the appearance. Further, in this case, the color of the print surface is partly or entirely dropped out, or restaining occur.
Furthermore, in domestic use, a back surface free from a detection mark as in photographic paper is preferred from the viewpoint of appearance. However, when no detection mark is provided, it is difficult to distinguish the image-receptive layer from the back surface. When the thermal transfer image-receiving sheet is set in a printer in such a state that the image-receiving surface and the back surface are inversive, the erroneous setting cannot be detected by the printer and the printer begins to print.
If that happens, in the conventional thermal transfer image-receiving sheet, fusing between the thermal transfer sheet and the back surface of the thermal transfer image-receiving sheet occurs within the printer, which inhibits the thermal transfer image-receiving sheet from being delivered from the printer, so that the printer should be sent to a maker for repair.
The provision of a dye-receptive layer on both surfaces of the substrate sheet is considered as a means for solving the problem of heat fusing of the back surface. In this case, however, when prints are put on top of one another for storage, the dye migrates to cause problems of a lowering in image density, staining of contact surface, restaining and the like. Furthermore, since the dye-receptive layer comprises a dyeable resin and is even, the image-receptive layers are likely to come into close contact with each other, which, also in the stage before printing, results in a problem of a failure in automatic feed such as a problem that a plurality of image-receiving sheets are carried together in an overlapped state in a feeder of a printer. For example, even though a filler is added to the image-receptive layer for the purpose of preventing the occurrence of this problem, the highlight portion of the print is likely to become unsharp.
Another means for solving the above problem is to add a release agent to the back surface layer as a dye-unreceptive layer. However, if the release agent is added in an amount sufficient to impart satisfactory releasability, the releasing component contained in the back surface layer is transferred to the image-receptive surface when the back surface layer is put on top of the image-receptive surface, which unfavorably raises problems of occurrence of a failure in printing such as partial dropout in the print portion and uneven print density, a lowering in coefficient of dynamic friction between the image-receptive surface of the image-receiving sheet and the transfer agent surface of the thermal transfer sheet, which is causative of the occurrence of a shear in the printing position of each color. Further, in this case, the releasing component contained in the back surface layer migrates to a feed and delivery mechanism, such as a paper feed rubber roller, and a platen rubber roller in a printer, which gives rise to a change in coefficient of friction of these members, so that troubles are likely to occur such as a failure in feed and delivery of sheets and oblique carrying of the image-receiving sheet.
Accordingly, an object of the present invention is to solve the above problems of the prior art and to provide a thermal transfer image-receiving sheet having excellent service properties for use in a thermal transfer system where a sublimable dye is used, which thermal transfer image-receiving sheet hardly causes a lowering in print density and migration of dye to the back surface of the image-receiving sheet when a plurality of image-receiving sheets are put on top of another for storage, can be delivered from the printer without fusing to the thermal transfer sheet by virtue of excellent releasability of the back surface even though printing is carried out on the thermal transfer image-receiving sheet with the image-receiving surface and the back surface being inversive and is free from an adverse effect of the release agent added to the back surface layer on the image-receiving surface and substantially free from the migration of the release agent to a sheet feed and delivery mechanism and a platen rubber roller.